In a conventional high-frequency heating apparatus, it is a general practice to separate high voltage carrying parts such as terminals of a high-voltage capacitor from a dead metal part such as an outer enclosure, which is accessible to a user, or to place an insulating material such as a plastic sheet on a surface of the dead metal part confronting the high voltage carrying parts where a sufficient space is not available for separation, in order to ensure electric insulation. FIG. 5 shows an example of such structures.
FIG. 5 depicts outer enclosure 1, high-voltage capacitor 2 and plastic sheet 3 made of an insulating material. Due to a limited space between terminals 2A of high-voltage capacitor 2 and outer enclosure 1, this structure ensures the electric insulation by placing plastic sheet 3 of the insulating material on a surface of outer enclosure 1 facing high-voltage capacitor 2.
There is another example of electric insulation between terminals of the high-voltage capacitor and a dead metal part such as the outer enclosure, as shown in FIG. 6, in which a thickness of an air guide is partially reduced to form a plastic hinge in a bottom side of it, and a portion of the air guide is bent downward. In FIG. 6, reference numerals 4 and 5 represent the air guide and a cooling fan respectively. High-voltage capacitor 2 is mounted under bottom side 4A of air guide 4, and end flap 4D is bent downward with plastic hinge 4B formed in air guide 4 so as to be located between terminals 2A of high-voltage capacitor 2 and the outer enclosure (not shown) to provide the electric insulation.
FIG. 7 shows air guide 4 of the prior art indicating a direction of separating a pair of molding dies, and FIG. 8 is a schematic illustration indicating a flow of cooling air in the vicinity of high-voltage capacitor 2 in a structure of the prior art, wherein reference numeral 6 represents a high-voltage transformer.
There is also another example of ensuring electric insulation between terminals of the high-voltage capacitor and the dead metal part such as the outer enclosure by mounting an insulation cover, as shown in FIG. 9. This example is disclosed in Japanese Patent Unexamined Publication No. H08-203664. In FIG. 9, insulation cover 8 covers high-voltage capacitor 2, high-voltage transformer 6 and magnetron 7 in a manner to isolate electrically the entire high voltage components from an outer enclosure (not shown).
However, in the conventional structure, which uses plastic sheet 3 to provide the electrical insulation as shown in FIG. 5, positioning of plastic sheet 3 depends on workmanship in a manufacturing process of the high-frequency heating apparatus. It is therefore necessary to make plastic sheet 3 larger in size in order to ensure the electrical insulation, taking into consideration that a deviation of certain extent is inevitable in the positioning. The increase in size of plastic sheet 3 makes the work of placing it difficult, and increases cost of the material. Besides, since plastic sheet 3 is stuck with glue or adhesive material, it is liable to fail the electrical insulation when it loses the adhesive property and comes off the position due to deterioration over time. In addition, because plastic sheet 3 is generally supplied as being backed with a liner of paper or film, it involves many working processes including such steps as removing plastic sheet 3, placing it on to outer enclosure 1, and disposing the liner as waste in the course of manufacturing the high-frequency heating apparatuses. This results in a considerable amount of waste matter at the same time.
On the other hand, in the structure having air guide 4 with plastic hinge 4B formed by partially reducing wall thickness in bottom side 4A as shown in FIG. 6, plastic hinge 4B cannot be bent directly downward at right angles because it has a tapered thickness to facilitate ejection from the molding dies, since the direction of separating the pair of molding dies of air guide 4 is in parallel with plastic hinge 4B, as shown in FIG. 7. If the molding dies are provided with a slide core or the like to avoid the tapered thickness, it makes a structure of the molding dies complex, shortens a useful life of the molding dies, and increases a labor and cost required for maintenance of the molding dies. Furthermore, since high-voltage capacitor 2 is disposed under bottom side 4A of air guide 4, as shown in FIG. 8, high-voltage capacitor 2 receives only a part of the cooling air reflected by high-voltage transformer 6. This also gives rise to a problem of substantially reducing cooling efficiency of high-voltage capacitor 2, because the cooling air reflected by high-voltage transformer 6 turns to hot air as it receives the heat of high-voltage transformer 6.
In addition, the conventional structure provided with separately mounted insulation cover 8, as shown in FIG. 9, increases a number of components due to the addition of insulation cover 8, and reduces working efficiency in the process of assembly. Moreover, since insulation cover 8 has a function of rectifying the cooling air, it needs to be considerably large in size, and therefore giving rise to another problem of increase in cost of the material.